Conventionally, one batch metering method is well known such that material stored in a hopper is dropped from a material outlet by opening a flap damper or a slide shutter provided at the lower part of the hopper and is received in a meter having a load cell or a mass measuring means of differential transformer type or of frequency type, and a material discharge is controlled to be stopped when the mass reaches a predetermined amount while executing a real time measurement.
According to this metering control method by mass metering, there is a height difference between the material outlet and the meter, so that the dropping material is further supplied in the meter after the mass metered by the mass measuring means reaches at a predetermined amount and the material outlet is closed. Therefore, in a prior metering control method, the mass of the dropping material depending on the opening degree and the kinds of material is measured in advance and the mass which is obtained by subtracting the value of residual dropping material is set as a target value.
However, the actual amount of residual dropping material is varied at each metering and the accidental error for the predicted value of residual dropping material often becomes large when the outlet is closed and its opening is specifically large. Therefore, a multistage metering is supposed in which the opening degree is made smaller just before material discharge is stopped.
According to such a multistage metering, a large amount of material is supplied by enlarging the opening degree of the material outlet at an initial supply stage and the opening degree is stepwisely reduced depending on the error caused by the value of dropping material just before stopping supply because the opening degree is small. Therefore, rapid metering can be achieved by an initial large amount metering and an accurate metering can be achieved by the final small amount metering.
In this system executing the multistage metering, a target supply mass switch value is determined as a set value at each stage, whether the supply amount reaches the set value is measured at each stage, and the stage is switched to next when it reaches the set value.
The following patent document discloses a metering method in which the supply amount is stepwisely changed at large, middle, and small.    Patent Document 1: Japanese Examined Patent Publication H06-12288-B